http://hallofbeorn.com/LotR/Details/Fire-filled-Trench-TFotW

If there are three of these locations in the staging area and a player uses one A test of Will, do the forced effects of each of the locations trigger, discarding the three of them? or only one of the forced effects will trigger? Leaving still two fire-filled trench in the stagging area?

Forced: When a player plays an event that would cancel the effects of a treachery card just revealed from the encounter deck, cancel the effects of that event. Then, discard Fire-filled Trench.

I think only one gets discarded.

All three Forced effects trigger (the triggering condition having been met). But you process one at a time, and I don't think the event's effects can be cancelled more than once. I think each additional attempt to cancel those effects fizzles, and after that the "Then," clause with its rules prevents the "discard Fire-filled Trench" from resolving.

I would have thought differently, but what makes me agree is that once the event is cancelled by one trench, that effect (the cancellation) is resolved and cannot be "cancelled" a second or third time.